In a vacuum exhaust device (vacuum pump) used for depressurizing and exhausting a device to be exhausted such as a vacuum chamber, it is general to connect a plurality of vacuum exhaust devices, which are different depending on the intended use, to one another in series such that gas can flow, thus achieving target performance. For example, a mechanical booster pump is adopted as a main pump for exhausting a device to be exhausted to an operating pressure and maintaining the pressure, and an oil rotary pump or a dry pump is adopted as a roughing pump for exhausting a vacuum system from an, atmospheric pressure to a pressure at which the main pump is capable of operating. Those vacuum pumps are used in combination, thus establishing a vacuum exhaust system with which target performance is achieved. The combination of vacuum pumps is not limited thereto and is diverse. There is a case where three or more vacuum pumps are combined.
In the case where such a plurality of vacuum pumps are combined, the respective vacuum pumps are generally arranged at proper positions and then connected to one another by a connection pipe or the like. For example, a connection structure in which each vacuum pump is fixed to a predetermined frame (installation base) and an exhaust port of the main pump and an intake port of the roughing pump are connected to each other with use of a pipe is generally used.
For example, Non-patent Document 1 below describes a vacuum exhaust system in which an exhaust port of an upper pump and an intake port of a lower pump are connected to each other with use of a pipe. Further, Non-patent Document 2 below describes a vacuum exhaust system in which vacuum pumps are installed on and within a frame and an exhaust port and an intake port of upper and lower vacuum pumps are connected to each other with use of a pipe.
Further, for vacuum pumps connected by the method as described above, widely used is a multistage roots vacuum pump having a multistage structure in which a space formed within a single casing is partitioned to form a plurality of pump chambers. In the multistage roots vacuum pump, it is general to connect pump chambers at respective stages in series (see Patent Document 1 below).    Patent Document 1: Japanese Patent Application Laid-open No 2002-364569    Non-patent Document 1: “Edwards vacuum product catalog Revision 3”. Edwards Japan Limited, p. 54    Non-patent Document 2: “Vacuum Technology and Innovative Ideas (ULVAC): Oil Rotary Vacuum Pump System, YM-VD/YM-VS Series (1580 L/min to 20000 L/min)”, [online], ULVAC, Inc., [retrieved on Apr. 16, 2010], Internet <URL: http://www.ulvac.co.jp/products/compo/F020006.html>